Partial oxidation processes for hydrocarbon feedstocks were developed and commercialised during the 1950's. The best known processes, such as the Shell gasification process and the Texaco gasification process, have been utilised in a number of commercial plants.
Such partial oxidation or gasification processes utilising hydrocarbon feedstock normally comprise three principal steps: (a) the actual partial oxidation or gasification step, in which the feedstock is converted into raw synthesis gas in the presence of oxygen and steam; (b) cooling the hot gases leaving the reactor; and (c) carbon and ash removal, in which residual carbon and ash contained in the reactor outlet gas is removed in a water wash, thereby obtaining an aqueous slurry comprising carbon and ash. The mixture of carbon and optionally ash is also referred to as soot. The amount of ash and carbon in the slurry will depend on the feedstock, operating conditions and on the type of gasification reactor used. The water content of the slurry is subsequently lowered by means of a dewatering process step.
A dewatering process is described in U.S. Pat. No. 5,628,911. This publication describes a dewatering of the soot slurry, to which a flocculant has been added, by means of a movable filter, consisting of a horizontally movable filterband followed by two vertically moving filterbands compressed by means of rollers pressing the filterbands together to exert a constant pressure until the water content of the soot filtercake is reduced to less than 80%. Thereafter the soot filtercake is released onto a movable conveyor belt, in the form of dry flakes or plates with an average thickness of about 2 mm.
U.S. Pat. No. 4,255,278 describes the dewatering of a slurry containing 14.9 wt % solids as obtained in a coal gasification process by means of a continuous centrifuge, model LB224 manufactured by Bird Machine Co. Inc. of South Walpole Mass. The process produced a cake containing 66 wt % solids and water containing 12 wt % solids. The water as obtained in this process cannot be considered to be clean. Furthermore, the soot particle properties of a coal gasification process differ substantially from the properties of the soot particles of a gasification process wherein the feedstock is a gaseous or liquid hydrocarbon as in the present invention. It is known that the solid particles in a coal gasification derived slurry have a greater size, i.e. having a Sauter-mean diameter (d50) of about 20 μm, than the solids as obtained in a mineral oil gasification derived slurry, which may have a d50 of about 2-8 μm. Furthermore the dry bulk density of the dry soot ‘coal’ particles is about 400 kg/m3 while the dry bulk density of the soot particles of the present process is between 5-15 kg/m3.
A disadvantage of the process of U.S. Pat. No. 5,628,911 is that by pressing the filterbands together water is obtained which contains considerable amounts of soot and ash. Furthermore a considerable amount of wash water is obtained when the filterbands are cleaned. These water streams must be treated before it can be disposed into the environment.